The present invention relates to a method and a device for the open-loop or closed-loop control of the braking action at at least one wheel of a motor vehicle.
A method and device for the open-loop or closed-loop control of the braking action at at least one wheel of a vehicle are known from the related art in a great variety of modifications.
For example, a method and a device for controlling the wheel slip are known from the publication xe2x80x9cFDRxe2x80x94Die Fahydynamikregelung von Boschxe2x80x9d [ESPxe2x80x94The Electronic Stability Program of Bosch] appearing in the Automobiltechnischen Zeitschrift (ATZ) 96, 1994, issue 11 on pages 674 through 689. Using this method and this device, respectively, the yaw rate of the vehicle is controlled by regulating the wheel slip. For this purpose, the measured yaw rate is compared to a setpoint value for the yaw rate, a system deviation for the yaw rate being ascertained. As a function of this system deviation, setpoint slip alterations, inter alia, are ascertained, from which the setpoint slip values to be adjusted at the individual wheels are determined. The actuators assigned to the individual wheels for adjusting the setpoint slip values are driven as a function of the setpoint slip values and the ascertained actual slip values. This type of control is designated as braking interventions carried out independently of the driver. As a backup, engine interventions can also be carried out to reduce the torque delivered by the engine. Primarily by the driver-independent, wheel-individual braking interventions, a yawing moment is applied to the vehicle by which the actual yaw rate of the vehicle approximates the setpoint value for the yaw rate. The contents of the publication xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d is herewith intended to be included in the description and thus to be part of the description.
The SAE paper 870337 xe2x80x9cASRxe2x80x94Traction Controlxe2x80x94A Logical Extension of ABSxe2x80x9d describes a method and a device for controlling the wheel slip. In the case of accelerative force, the traction slip of the driven wheels is controlled with this method and this device, respectively. To this end, the ascertained actual slip is compared to appertaining slip thresholds. If the actual slip exceeds the slip thresholds, then first of all, driver-independent braking interventions are carried out at the driven wheels. Secondly, the torque delivered by the engine is reduced. The contents of the SAE paper 870337 is herewith intended to be included in the description and thus to be part of the description.
The German Patent 196 04 126 describes a control of a braking system, in which the braking pressure is built up and reduced by pulses with variable parameters. The parameters are altered as a function of the dynamics of the pressure change, particularly as a function of the temperature of the brake hydraulic fluid.
Both in the closed-loop control as well as in the open-loop control of the wheel slip indicated above, the following problem can occur: Transverse forces act on the vehicle during cornering. The result of these transverse forces is, inter alia, that in the case of the wheel brake cylinders allocated to the wheels, the pistons, which are moved for generating the braking action, are deflected. This deflection of the pistons leads to an air gap. If in such a situation, in which the pistons are deflected because of cornering, the intention is now to carry out a braking intervention independently of the driver, then an increased demand for braking medium is required to compensate for this increased air gap. If this increased need for braking medium is not taken into account when carrying out a driver-independent braking intervention, then especially in the control of the wheel slip indicated at the outset, by which the yaw rate of the vehicle is controlled, deviations can occur in the control.
Various devices and methods which deal with this problem are known from the related art.
For example, reference is made to the traction control system disclosed in EP 0 166 258 B2. In this traction control system, below the thresholds signaling the incipient slippage, a small braking pressure is applied automatically at the brakes which is just large enough that the brake shoes are in fact applied, but still no considerable braking takes place. In this case, it is a question of a fill pulse of constant duration that is fed prior to the actual pressure feed which originates in the traction control system and with which the traction slip, which is too high, is to be eliminated.
The German Patent 196 15 294 describes a slip control in which a fill pulse is likewise ascertained. To this end, first of all a criterion describing and/or influencing the vehicle movement is ascertained, and secondly, a quantity is ascertained describing the wheel dynamics of at least one wheel. As a function of the ascertained criterion, it is checked whether a driver-independent braking intervention is foreseeable at one wheel. If it is determined that a driver-independent braking intervention is foreseeable, then prior in time to the foreseeable driver-independent braking intervention, the actuators assigned to the wheel are actuated slightly for a variable time duration. The duration of the slight actuation of the actuators is ascertained as a function of the quantity describing the wheel dynamics of the appertaining wheel.
The two slip controls belonging to the related art have the disadvantage that a separate fill pulse is generated which is not due to the slip control. It can thereby occur that a fill pulse is generated, although a pressure feed which originates in the slip control is not necessary at all. This can adversely affect driving comfort, since to generate the fill pulse, a pump contained in the braking system must be actuated, which is associated with a development of noise perceptible by the driver.
Therefore, the object of the present invention is to improve existing methods and devices for the open-loop or closed-loop control of the braking action at at least one wheel of a vehicle.
The method of the present invention is a method for the open-loop or closed-loop control of the braking action at at least one wheel of a vehicle. In this method, a transverse-dynamics quantity is ascertained which describes the transverse dynamics of the vehicle. This transverse-dynamics quantity can be the transverse acceleration acting on the vehicle or the yaw velocity of the vehicle. Provided that appropriate sensors are present, these quantities are either ascertained with the aid of these sensors, or, if appropriate sensors are not present, are derived from other quantities, for example, the wheel speeds.
As a function of a vehicle-dynamics quantity which describes the vehicle dynamics, or a wheel-dynamics quantity which describes the wheel dynamics of at least one wheel, it is determined whether a driver-independent braking intervention is necessary.
For example, if it is a question of a slip control as is described in the publication xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d indicated at the outset, then the vehicle-dynamics quantity corresponds to the yaw velocity of the vehicle. In altered form, the transverse acceleration acting on the vehicle can also be evaluated as an alternative. At this point, it should be pointed out that the use of the two terms transverse-dynamics quantity and vehicle-dynamics quantity are to be understood such that both quantities are fundamentally different quantities. However, this is not intended to rule out the possibility that both quantities are identical. For example, this is the case when both the transverse-dynamics quantity and the vehicle-dynamics quantity correspond to the transverse acceleration acting on the vehicle.
A driver-independent braking intervention is necessary when a system deviation for the vehicle-dynamics quantity exists, or when the system deviation is greater than a corresponding threshold value.
For example, if it is a question of a device as is described in the SAE paper 870337, then the wheel-dynamics quantity corresponds to the wheel slip. Alternatively, the wheel deceleration or the wheel acceleration can also be evaluated. The combined evaluation of a vehicle-dynamics quantity and a wheel-dynamics quantity is also conceivable. In this case, a driver-independent braking intervention is performed when the wheel-dynamics quantity is greater than a corresponding threshold value.
In the event that a driver-independent braking intervention is necessary, a pulse-shaped signal is determined for triggering the actuators assigned to at least one wheel. This pulse-shaped signal is advantageously a modulated, particularly a pulse-width-modulated signal. The degree to which the actuators assigned to the respective wheel are actuated is established by the modulation of the signal. Usually the actuators are valves allocated to the respective wheel, by whose actuation braking medium is let into and out of the wheel-brake cylinder.
For example, the braking system can be a hydraulic or an electrohydraulic braking system, in which a brake fluid is used as braking medium. Alternatively, the braking system can also be a pneumatic or an electropneumatic, in which air is used as braking medium. In addition, it can be an electromechanical braking system, in which the braking action is applied in an electromotive manner.
According to the present invention, the time duration of the first pulse of the pulse-shaped signal is influenced as a function of the transverse-dynamics quantity. Because the first pulse of the pulse-shaped signal is influenced in its duration, no separate fill pulse is necessary. That is to say, no additional triggering of the pump required. The increased air gap is compensated directly by the triggering of the actuators with the aid of the pulse-shaped signal.
Advantageously, the duration of the first pulse of the pulse-shaped signal is prolonged as a function of the transverse-dynamics quantity.
Because the first pulse of the pulse-shaped signal is prolonged, a pressure-buildup dynamic is achieved when cornering which is comparable to that when driving straight ahead.
In one advantageous refinement, the braking action is applied hydraulically, and the pulse-shaped signal is determined as a function of at least two different, selectable determination modes. In this case, it is provided in particular that the determination modes are based on at least two different hydraulic models. The determination modes are then selected as a function of the transverse-dynamics quantity, such that the first pulse of the pulse-shaped signal is influenced in its time duration.
Further advantages and advantageous refinements can be gathered from the Drawing and the description of the exemplary embodiment.